Active-matrix LCD devices including active elements such as TFTs as switching devices have been increasingly used. Materials used for the semiconductor layers of the TFTs in the LCD device include polysilicon (p-Si) and amorphous silicon (a-Si), for example. A TFT having a polysilicon layer is herein referred to as a p-Si TFT whereas a TFT having an amorphous silicon layer is herein referred to as an a-Si TFT. Comparing both the TFTs against each other, the a-Si TFT has advantages of a smaller number of fabrication steps and allowing lower-temperature fabrication steps.
FIG. 13 shows the sectional view of an LCD panel in a general LCD device, wherein a-Si TFTs 230 are fabricated. The LCD panel 200 includes a TFT substrate 202, a counter substrate 204, and an LC layer 203 sandwiched therebetween. Each substrate 202 or 204 includes an orientation film 207 between the same and the LC layer 203. A backlight unit 206 is disposed at the rear side (bottom side) of the TFT substrate 202 with an intervention of a polarizing plate 201, for irradiating the LCD panel 200 through the polarizing plate 201. Another polarizing plate 205 is disposed at the front side (top side) of the counter substrate 204. The polarizing plates 201 and 205 have respective polarizing axes which are perpendicular to each other.
FIG. 14 shows the top plan view of a portion of the TFT substrate 202 shown in FIG. 13, as viewed from the counter substrate 205. The TFT substrate 202 includes thereon a plurality of signal lines 231, a plurality of scanning lines 232 extending perpendicular to the signal lines 231, and a plurality of TFTs 230 each formed in the vicinity of the intersection between one of the signal lines 231 and one of the scanning lines 232 for driving a pixel electrode 229. Each TFT 230 has a drain electrode 225 extending from a corresponding signal line 231, a source electrode 226 connected to the pixel electrode 229, and a gate electrode 222 extending from a corresponding scanning line 232.
FIG. 15 shows one of the TFTs 230 shown in FIG. 14 in a sectional view taken along line XV-XV in FIG. 14. The TFT shown in FIG. 15 can be fabricated by a known technique such as described in JP Patent Publication 3152193.
In general, the p-Si TFT has a coplanar structure wherein the gate electrode, source electrode and drain electrode are formed on one of both the surfaces of the polysilicon layer, whereas the a-Si TFT has a staggered structure wherein the gate electrode 22 opposes the source electrode and drain electrode with an intervention of the a-Si layer 234, as shown in FIG. 15. The staggered structure shown in FIG. 15 is referred to as an inverted staggered structure due to the gate electrode 222 being disposed at the bottom side of the a-Si layer 234. If the gate electrode is disposed at the top side of the a-Si layer instead, the staggered structure will be referred to as a non-inverted staggered structure.
The drain electrode 225 is in contact with the a-Si layer 224 via an ohmic contact layer 233a, whereas the source electrode 226 is in contact with the a-Si layer 224 via an ohmic contact layer 233b. The source electrode 226 is connected to the pixel electrode 229 via a through hole 228. The TFT 230 shown in FIG. 15 is generally called channel-etched TFT. It is to be noted that the drain electrode 225 has a planar size equal to the planar size of the underlying ohmic contact layer 233a, whereas the source electrode 226 ha a planar size equal to the planar size of the underlying ohmic contact layer 233b. 
The a-Si layer 224 in the TFT 230 includes a channel region 234 which overlies the gate electrode 222 and extends from the inner edge of the ohmic contact 233a underlying the drain electrode 225 to the inner edge of the ohmic contact 233b underlying the source electrode 226. The length of the channel region 234 is “L” as shown in the figure. In the TFT 230 of the inverted staggered structure, the gate electrode 222 acts as a light shield film, which shields the channel region 234 against the light emitted from the backlight unit 206.
FIG. 16 shows an enlarged top plan view of the TFT 230. Each of the drain electrode 225 and source electrode 226 has an inner edge opposing the edge of the other of the drain electrode 225 and source electrode 226 with an intervention of the channel region 234. The channel length “L” between the drain electrode 225 and the source electrode 226 is constant independently of the widthwise position of the channel region 234, as shown in FIG. 16.
FIG. 17 shows a top plan view of a portion of the counter substrate 204 opposing the portion of the TFT substrate 202 shown in FIG. 14. As understood from FIGS. 14 and 17, the counter substrate 204 includes a black matrix 242 having a pattern overlapping the TFT 230, signal line 231 and scanning line 232 on the TFT substrate 202, as viewed in the direction of the transmission of the backlight. More specifically, the black matrix 242 shields the TFT 230 etc. against the light from the backlight unit 206, and passes part of the light to thereby define effective pixel areas, or light transmission areas 245. The intensity of the light transmitted through each light transmission area 245 is controlled by the voltage applied through a corresponding TFT 230 between a corresponding pixel electrode 229 and the counter electrode 244 (FIG. 13) on the counter substrate 204. Each light transmission area 255 is provided with a R, G or B coloring layer 243 to display a color image on the screen of the LCD panel.
Although the black matrix 242 is made from a material having a lower reflectance, part of the light incident onto the counter substrate 204 from the backlight unit 206 is reflected by the black matrix 242 to return to the TFT substrate 202. Some of the part of light returned to the TFT substrate 202 is reflected for a multiple of times by the gate electrode 222 and the drain electrode 225 or source electrode 226, to enter the channel region 234 of the TFT 230, especially in a larger amount into both the edge portions of the channel region 234. The light incident onto the channel region 234 causes leakage current across the channel region 234 to degrade the switching characteristic of the TFT 230, whereby the LCD device suffers from degradation of the image is quality.
For prevention of the leakage current caused by the light incident onto the channel region 234, a technique is known wherein the black matrix 242 and the coloring layer 243 are formed on the TFT substrate 202 instead of forming the same on the counter substrate 204. In this technique, reduction of the distance between the black matrix 242 and the TFT 230 allows the light incident onto the channel region 234 to be reduced, thereby reducing the leakage current in the TFT 230. However, this technique involves a problem in that TFTs 230, black matrix 242 and coloring layers 243 must be consecutively formed on the TFT substrate 202, which is difficult to achieve.
The channel region 234 of the TFT is also subjected to the incident light from the backlight unit 206 through the rear side of the TFT substrate 202, in addition to the reflected light from the black matrix 242 as described above. This is because the gate electrode 222 cannot completely shield the channel region 234 against the incident light. In particular, one of the edge portions of the channel region 234 in the vicinity of the pixel electrode 229 is subject to a larger amount of light compared to the other of the edge portions of the channel region 234 in the vicinity of the scanning line 232 because the scanning line 232 extends apart from the channel region 234. This causes a larger leakage current in the vicinity of the one of the edge portions of the channel region 234. It is to be noted that the gate electrode 222 has an extended portion in the vicinity of the pixel electrode 229 for prevention of the incident light, as will be understood from the length “d” of the extended portion of the gate electrode 222 shown in FIG. 16.
The extended portion of the gate electrode 222, however, reduces the light transmission area 255 of the pixel to degrade the brightness or luminance of the pixel because the extended portion enlarges the TFT area and thus the blocking area of the black matrix 242 which shields the TFT area against the incident light.